1. Field of the Invention
The invention relates to a method of selectively obtaining phosphorus from heavy metal- and phosphate-containing solid material.
2. Description of Related Art
Phosphorus is a limiting nutrient to the growth of plants, and as it exclusively occurs in bound form, it is mined in the form of phosphate ore from deposits which are exhaustable in the form presently known. Mined phosphates are mainly processed to form plant available mineral fertilizer.
The treatment of waste water from household or industry results in the generation of sewage sludge which may be utilized in agriculture because of the nutrients nitrogen and phosphorus contained therein. As the sewage sludge does, however, also contain pollutants such as heavy metals, e.g. lead and cadmium, agricultural utilization of sewage sludge is increasingly challenged. Attempts are accordingly undertaken to largely clean phosphates contained in the sewage sludge from heavy metals by selective recovery.
Phosphorus is removed or eliminated from waste water in undissolved form only, wherein the following biological and chemical-physical methods may be differentiated.
In chemical-physical phosphorus removal, the dissolved phosphate is precipitated by the addition of precipitating agents. As precipitating agents, Fe3+, Al3+, Fe2+ and Ca2+ are predominantly used. Disadvantageously, however, the precipitating agents partly are by-products or waste products of commercial processes and, therefore, contain impurities such as, e.g., heavy metals and organic halogen compounds, which increase the pollutant load of the sewage sludge. Ferrous phosphates, moreover, cannot be taken up by plants. The use of precipitating agents in pure form, on the other hand, is expensive.
For an adequate recovery of phosphorus from solids, such as sewage sludge incineration ash, it is necessary to solubilize chemically bound phosphorus. Solubilizing chemically-physically bound phosphorus to a large extent, however, is only possible using acids and at low pH values. As a rule, solubilization of phosphate is carried out by acidic decomposition using mineral acid, followed by selective precipitation steps. In the Seaborne process, for example, at first an anaerobic sludge treatment is carried out in which the bound phosphorus is subsequently solubilized along with the heavy metals by the addition of acid. The pH is then raised again and the heavy metals are precipitated selectively using H2S and removed. In a further step, phosphorus is selectively precipitated by adding bivalent metals. These processes, however, are time-consuming and costly.
Enhanced biological phosphorus removal utilizes the capability of polyphosphate-storing or polyphosphate-accumulating microorganisms, in particular bacteria, to accumulate phosphorus as an energy-rich polyphosphate in the form of granula. This process is known as the Bio-P process and is widely used in waste water disposal for the removal of dissolved phosphate. Processes for enhanced biological phosphorus removal are described, e.g., in DD 282 902 A5, DE-A-36 02 736 A1, DE 196 35 391 A1, GB 2 351 284 A, and DE 10 2005 007 408 A1. The phosphorus removed from the waste water stream by biological phosphorus removal is eliminated from the system together with the excess sludge.
Enhanced biological phosphorus removal, however, is not directly applicable for the recovery of chemically bound phosphorus. In phosphorus-containing solids such as sewage sludge incineration ash, however, phosphorus is at least partly present in a chemically bound form. In sewage sludge incineration ash, the phosphorus fully remains as a residue in the ash. Although heavy metals such as Pb, Cd, Cu, Cr, Hg, Ni, and Zn as a rule are present in the ash only as trace elements, the maximum permissible values set in the Rules and Regulations for Fertilizers may nevertheless be exceeded. This necessitates further treatment of the ash. In addition, absent of further treatment, the availability of phosphorus, which in the ash is predominantly present in the form of apatite, is not sufficient in the soil for nutrient supply of plants.